JP-A-2015-206764 discloses a technology for measuring a position of an eye of an occupant by using a monocular camera. The technology disclosed in JP-A-2015-206764 relates to a head-up display (HUD) device, and is configured such that the position of the eye of the occupant sitting on a seat device of a vehicle can be measured, and a display position of a display can be adjusted based on the position of the eye of the occupant. In the HUD device disclosed in JP-A-2015-206764, a depth distance (a distance from the monocular camera to a photographic object) of an image in which a face of the occupant is photographed is estimated based on a space dimension between left and right eyes of the occupant photographed by the monocular camera. That is, the depth distance of the image is estimated based on the fact that the space dimension between the left and right eyes increases as the occupant gets closer to the monocular camera, and the space dimension between the left and right eyes decreases as the occupant gets away from the monocular camera. Then, the position of the eye of the occupant is obtained by the depth distance and the image in which the face of the occupant is photographed.
In the above-described method for measuring the position of the eye of the occupant, the depth distance of the image is estimated based on the space dimension between the left and right eyes of the occupant photographed by the monocular camera. However, there are individual differences in space dimension between left and right eyes of the occupant, and it is difficult to accurately obtain the depth distance of the image according to the space dimension between left and right eyes. Therefore, there is a problem that measurement accuracy of the position of the eye of the occupant is low. Here, for example, it is possible to accurately measure the depth distance of the image by using a stereo camera, or the like. However, since two cameras are required, the cost increases.